Ultra-high molecular weight polyolefins exhibit excellent shock resistance, abrasion resistance, resistance against chemicals, tensile strength and the like properties compared to those of general-purpose polyolefins, and are finding spreading applications as engineering resins. However, products having decreased thicknesses obtained by using ultra-high molecular weight polyolefins exhibit inferior secondary machinability such as heat-sealing property to those of the general-purpose polyolefins.
In order to improve such a problem of the ultra-high molecular weight polyolefins, it has been attempted to laminate a layer of a thermoplastic resin having good heat-sealing property.
For example, Japanese Patent Publication No. 108138/1983 discloses a process for producing an inflation film by using a generally used extruder in order to obtain a laminate of a high molecular weight polyolefin. According to this publication, a film is formed by using a material having a weight average molecular weight of from 800,000 to 1,500,000 and a melt flow index (MFR) of from 0.03 to 0.1. Relying upon this method, however, it is difficult to laminate an ultra-high molecular polyolefin having an MFR of smaller than 0.03 and a limiting viscosity [.eta.] of not smaller than 5 dl/g on other polymer.
The present applicant has previously proposed a process for producing an inflation film by using a tube die of which the mandrel revolves accompanying the revolution of the screw of an extruder, in order to obtain, from an ultra-high molecular weight polyolefin alone, a formed article in which the decreasing of the molecular weight is suppressed to be as low as possible (Japanese Patent Publication No. 55433/1994).
According to this process, however, the tubular film that is extruded faces sideways arousing a problem in that the upper portion of the tubular film becomes thin, the lower portion thereof becomes thick, developing a difference in the thickness between the upper portion and the lower portion. In order to increase the rate of production, furthermore, the revolving speed of the mandrel must be increased accompanying the revolution of the screw, giving rise to the occurrence of a problem in that the resin is deteriorated due to the friction. There further remains a problem in that the mandrel must be lengthened in order to eliminate flight marks on the resin.
In order to produce a laminate of an ultra-high molecular polyolefin, it has also been attempted to laminate other materials by using a skived film obtained by skiving a molded article of the ultra-high molecular weight polyolefin (Japanese Laid-Open Patent Publication No. 173505/1996). In a step of obtaining a skived film by using a blade, however, vertical streaks formed by the blade tip causes the surface to become rugged. Therefore, a laminate of the skived film and other resins still fails to give a satisfactory result when it is used for a variety of applications. Furthermore, limitation is imposed on the width and length of the laminate that is obtained, which is not favorable even from the standpoint of productivity.
Referring to the laminate of a skived film of an ultra-high molecular polyolefin and other resins disclosed in the above Japanese Laid-Open Patent Publication No. 173505/1996, the ultra-high molecular polyolefin layer constituting the laminate has not been oriented. This is because, the cylindrical molded article which will be skived to obtain a skived film is obtained by heat-melting the ultra-high molecular polyolefin.
There can also be quoted a method of producing a multi-layer laminate in which the individual layers are oriented, by laminating a single oriented film of an ultra-high molecular polyolefin and an oriented thermoplastic resin film under the application of a pressure between a pair of rollers heated at a predetermined temperature. In the multi-layer laminate obtained according to this method, however, the resins constituting the individual layers are once melted and, then, adhered in an interface where the layers are to be adhered together, and, hence, become amorphous. Therefore, the multi-layer laminate obtained according to this method is different from the multi-layer laminate of the present invention.
Furthermore, when it is attempted to laminate an ultra-high molecular polyolefin on another material, use of a skived film imposes limitation on the width. Therefore, limitation is imposed on the coating width and on the length. Despite it is attempted to continuously produce a laminate with another material, therefore, limitation is imposed by the length of the skived film, and the roll must be replaced every after the end of the roll of the skived film, causing inconvenience from the standpoint of production.
In trying to improve abrasion resistance on the surface of the pipe by adhering such a laminate on the cylindrical surfaces of a polyolefin pipe or a metal pipe, no seam is formed on the surfaces of the pipe by an ultra-high molecular polyolefin multi-layer laminate if a tubular laminate could be stuck to the polyolefin pipe or to the metal pipe, which is very effective from the standpoint of coating and productivity. So far, however, there was available no ultra-high molecular polyolefin multi-layer laminate tube in which the ultra-high molecular polyolefin layer has a molecular weight greater than a predetermined value and the whole laminate exhibits excellent mechanical strength.
It has therefore been desired to provide a multi-layer laminate which includes an ultra-high molecular polyolefin layer on which less limitation is imposed, exhibiting excellent mechanical properties such as tensile strength, film shock resistance, etc.
It is further considered that the ultra-high molecular polyolefin that is stuck on a metal plate can be used for the applications where strength and surface abrasion resistance are required, such as industrial conveyer lines. When used for such applications, however, the laminate poorly adheres and is not practically utilizable unless it adheres to the metal plate or to the metal foil at a low temperature maintaining a large strength. When it is attempted to adhere the laminate at a high temperature, furthermore, the laminate of the ultra-high molecular polyolefin and the metal plate is warped. In addition, when used as an industrial conveyer line, the surfaces of the line are broken in short periods of time and durability is not maintained for extended periods of time unless the multi-layer laminate stuck to the metal plate has a sufficiently large strength.
It has therefore been desired to provide a multi-layer laminate including an ultra-high molecular weight polyolefin that can be adhered to the metals, exhibiting a sufficiently large adhesion strength at low temperatures and excellent mechanical strength.
It has further been desired to produce such a multi-layer laminate efficiently.